1. Field of the Invention
The present invention relates to a spark plug with igniting portion chip composition for use in internal combustion engines.
2. Description of the Related Art
Conventional spark plugs used in internal combustion engines, such as automotive engines, have an igniting portion formed of a platinum (Pt) alloy chip welded to a tip end of an electrode in order to improve its resistance to spark consumption. However, in view of the high cost of platinum, it has been proposed to use less expensive iridium (Ir) as a chip material.
A problem with the use of Ir as a material for the igniting portion of the spark plug is that Ir easily oxidizes and evaporates in a high temperature range of 900 to 1,000.degree. C. Therefore, if it is directly used in the igniting portion of the electrode, it is consumed by oxidation and evaporation to a greater extent than by sparking. In order to retard the oxidation and evaporation of Ir, it has been proposed that a material having a rare earth oxide, such as Y.sub.2 O.sub.3, dispersed in Ir, should be used as added to the chip composition (See Unexamined Japanese Patent Publication (kokai) No. Hei. 7-37677). However, as recent engine models are designed to increase output power, the range of temperatures over which the spark plug is used tends to shift towards the higher end, and even the spark plug that uses a chip made of the proposed material may not have a satisfactory durability.